The invention relates to a wrist prosthesis having a first, proximal section and a second, distal section. The proximal section connects with the distal end of the radius bone and the distal section with at least one, especially with two, metacarpal bones.
U.S. Pat. No. 5,458,646 shows a wrist prosthesis in which a first, proximal section is connected rigidly with the distal end of the radius and the distal end of the ulna. A second, distal section of the wrist prosthesis is connected with the carpal bones. This second, distal section of the wrist prosthesis has a rounded, proximal end, which abuts on a straight surface of the proximal section. Such a wrist prosthesis has a number of disadvantages. For one, it connects the radius rigidly with the ulna and this limits movement of the wrist in the pronation-supination direction. Additionally, the connection of the distal section of the wrist prosthesis with the carpal bones is inadequate. Finally, such a wrist prosthesis is only usable in cases where osteoporosis has not yet progressed too far, so that it is only conditionally usable or installable.
An object of the present invention is, therefore, to provide a wrist prosthesis that has a wider range of applicability.
This object is achieved with a wrist prosthesis having a first, proximal section and a second, distal section, wherein the proximal section is connected with the distal end of the radius and the distal section is connected with at least one, especially with two, metacarpal bones, and each of the sections contains one part of a curved bearing.
The wrist prosthesis of the invention can be used advantageously in the case of rheumatoid arthritis. It has the essential advantage that the proximal section is connected rigidly only with the radius. A single anchor serves for this purpose. The anchor is cemented relatively far into the radius. Moreover, the second, distal section of the prosthesis is connected with at least one, especially with two, metacarpal bones, this providing significantly better stability and anchoring. For this purpose, screws are screwed into the metacarpal bones. The screws have a relatively long, thread-free screw tip, which is rounded on the front end. Using this relatively long screw, a secure connecting of the distal section with these bones is assured. Finally, each section exhibits one part of a curved bearing, to assure an optimal support both in the axial direction as well as in all transverse directions.
The anchor preferably has an asymmetric shape and an anchor head extending above the radius bone. This anchor head forms one part of the curved bearing.
In a preferred embodiment, the distal section has a carpal plate, which seats the connecting elements, especially bone screws, which connect to the bones of the hand. The carpal plate has seats for the heads of the screws. Connecting the screws using a carpal plate has the advantage that the distal section of the wrist prosthesis has a relatively stable construction, so that the metacarpal bones experience an optimal support. A stiff and secure connection of the distal section with the hand is achieved. In this way, a loosening is made highly unlikely.
For fitting the wrist prosthesis to different wrist sizes, the seats are expanded essentially lengthwise and in each case have at least two seating positions for the screw heads. These seating positions allow the positions of the screws to be matched to the positions of the metacarpal bones, without necessitating provision of differently configured carpal plates. The carpal plate is thus universally installable.
In a preferred embodiment, the seats for the screws have the form of a figure 8. This makes it possible to have different positions for the screw heads and the separation of the screws from one another becomes adjustable.
In a further, preferred development of the present invention, it is provided that the axes of the seats are inclined in the plane of inclination of the wrist by an angle of 5-degrees to 15-degrees, especially 12-degrees, relative to the axis of the proximal section. The wrist is thus inclined by an angle in the range of 5-degrees to 15-degrees, with this inclined position representing the basic position.
Advantageously, the carpal plate is connected with the proximal section through a bearing element. This bearing element contains the other part of the curved bearing and its size can be fitted to that of the wrist. The different bearing elements thus permit a relatively accurate matching of the wrist prosthesis to the size of the wrist. The bearing element is connected with the carpal plate, for example, using a screw connection, or the bearing element has a dovetail connection, by which it is connected with the carpal plate. The dovetail connection can be provided with a snap-lock connection, so that the coupling of the bearing element to the carpal plate is quick and, above all, accomplished without tools.
In a preferred embodiment of the invention, present the bearing element is provided with a lug, which extends into a groove on the anchor head, or vice versa. This stabilizes the bearing in the pronation-supination direction, while inclination of the bearing remains unimpeded.
A further development provides that the anchor head is shaped on its pin side for reception of an ulna-radius connecting means, especially an ulna plate. If necessary, the anchor can be equipped with this ulna plate. A connection of the radius with the ulna can be produced using this ulna plate. To accomplish this connection, the ulna plate extends laterally beyond the anchor head of the anchor and has a seat for an ulna screw, which is screwed into the end of the ulna.
An insert is provided in the seat for the ulna screw, in order that the ulna screw can have a movable connection with the ulna plate. In this way, it is assured that, despite the connection of the ulna and radius, relative movement between these bones is still possible, as was the case originally. Thus, the wrist is significantly more flexible in the pronation-supination direction.
The ulna screw is journaled in the insert such that it can swivel and/or undergo axial displacement. This movable journaling allows, additionally, a relatively unimpeded rotation of the wrist, but, despite the relatively large freedom of the ulna, it is supported afterwards as before.
Wear resistance in the bearings is achieved by making the bearing element and/or the insert from plastics material, especially polyethylene (PE). The other components can be of steel, and especially titanium, the surface of which is provided with a coating which serves as a substrate promoting bone growth. An example of such a known coating is referred to as HA-coating, where HA stands for hydroxy apatite.
A problem-free movement of the wrist is made possible by providing the curved bearing with a circular arc curvature in both the pronation-supination direction and in the inclination direction.
A high installability for the wrist of the present invention is achieved by forming the proximal and/or the distal section of the wrist prosthesis from modularly combinable components, which, if necessary, can have different sizes. The separate modules can be assembled on site, i.e. during the operation. These can e.g. be differently thick and/or differently long bone screws, differently sized carpal plates, differently sized bearing elements, differently sized ulna plates, and differently thick and long anchors, as well as differently sized ulna screws.